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Increasing regulation and costs of soil fumigation to manage diseases such as
Verticillium wilt caused by soil-borne pathogens have prompted a search for
management alternatives. Single-year microplot experiments were conducted in 2002
and 2003 in central Oregon to evaluate the effects of Austrian winter pea, broccoli,
and Sudan grass green manures applied at three rates (6, 12, or 24 Mg h⁻¹) on soil and
potato root populations of Verticillium dahliae, severity of Verticillium wilt of potato,
and tuber yield. Amendment rate had no effect on inoculum density (ID) or root
infection (RI), but affected relative area under the senescence progress curve
(RAUSPC). Green manures of broccoli applied at all three rates, Sudan grass applied
at 6 or 12 Mg ha⁻¹, and Austrian winter pea applied at 24 Mg ha⁻¹ reduced soil
populations of V. dahliae compared to the unamended control. For green manures of
broccoli or Sudan grass, reduction in ID could partially account for reduction of
RAUSPC (r = 0.44 and 0.46, respectively). Green manures of Austrian winter pea
applied at 12 or 24 Mg ha⁻¹ and broccoli or Sudan grass applied at 24 Mg ha⁻¹ reduced
RAUSPC by 74, 70, 70, and 52%, respectively, compared to the unamended control.
None of the green manure treatments in either experiment was able to significantly
increase yield relative to the unamended control. However, in 2002, Sudan grass
applied at 12 and 24 Mg ha⁻¹, and broccoli applied at 24 Mg ha⁻¹ resulted in a mean different from that of the uninfested plots and yield was negatively correlated with
disease severity (r = -0.61, P ≤ 0.0001). In 2003, due to an earlier harvest, there were
no differences in yield between amended and unamdended plots and no relationship
between yield and disease severity.
To explore possible mechanisms of suppression, soil chemical and microbial
properties related to severity of Verticillium wilt were identified using regression
analyses. We then assessed how these soil properties were affected by the green.
manure treatments. Soil population of V dahliae predicted 48 and 52% of the
variability in disease severity in 2002 and 2003, respectively. In 2002, after
accounting for different levels of ID, inclusion of a tenn for pH improved goodnessof-
fit of the regression model to 60%. Severity of Verticillium wilt was
overwhelmingly influenced by a wide soil pH gradient (pH 5.2 to 7.5) across the 2002
field. Soil pH was positively related to disease severity and was not affected by green
manure treatments. In 2003, after accounting for different levels of ID, inclusion of a
term for NO₃-N, microbial biomass (MBC), or microbial respiration (MR), improved
fit of the model to 64, 66 or 62%, respectively. All soil properties identified in 2003
were negatively related to disease severity and were positively associated with green
manure amendment rate. Finally, stepwise regression using data pooled from both
studies resulted in a model which included terms for ID, soil pH, and FDA and
accounted for 63% of the variability in RAUSPC.